Allow git-version.h to be provided for the build

Allow git-version.h to be provided for the build

For a non-git build or for a git build w/o adequate git history, we now
allow the git-version.h file to be provided before the build.  If the
file does not exist, we either create an empty file or put a define of
RSYNC_GITVER in it based on the output of git describe.  The github
builds now snag the git-version.h file that was generated for the last
commit so that they all get the same version string, even with a shallow
checkout.